I got a new epiphone es 339 yesterday and I have noticed there is a strange almost reverb sound when I play all the strings. For example if i play a slow strumming pattern with a open g the sound seems to echo and build up. This does not occur if I pluck the strings only when strumming. The amp has no reverb effect on and sounds completely fine when I play with any other of my guitars. I am new to using humbucker pickups is it something to do with that ? Any help would be appreciated
That's acoustic feedback. Humbuckers are comparatively sensitive to string movement while comparatively inert towards electric signals. If you get close to an amp with single-coil pickups and/or mute the strings by hand, you get electric feedback which is a pretty violent howl/screech.
With Humbuckers, you are more likely to get acoustic feedback where the amp produces enough volume to keep the guitar strings vibrating longer than if they would not be in the vicinity of the amp.
Things to test: get closer carefully (you don't want electric feedback to start) and see whether the strings start vibrating on their own. Turn your back (and thus the guitar's) to the amp and see if the effect stops.
Turn the volume down and up and see how it affects the effect.
It's just a sign that your pickups are sensitive, your guitar has good sustain on the strings (acoustic guitars won't do that easily since they emit a lot of energy via acoustic sound), and your amp produces so little noise of its own that you are comfortable turning it up.
Some people build this into their play. It's easily controlled by putting distance between you and the amp, and it's a sign that you are working with good equipment. Long sustain is nice for both clean and distorted play.
Without hearing the effect you speak of, it's difficult to say for sure, but I'll hazard a guess. The literature I've read on this particular model of guitar speaks of a desirable feature called improved sustain. That means that less energy from the vibrating strings is lost into the body of the guitar allowing the strings to vibrate for a longer period of time. That's a good thing usually, but it can also make a guitar more susceptible to problematic feedback when the strings of a guitar start "hearing" the sound from the amplifier and begin vibrating sympathetically. This vibration is picked up by the guitar's pickups and amplified creating what sounds like a runaway guitar chord or note. The usual solution for this is to position yourself and the guitar away from the amplifier or turn down the volume of the amp making it more difficult for the guitar strings to "hear" the amplifier sound output. Another solution is to adjust the tone with an EQ to minimize the offending frequencies. You will probably learn by trial and error just how far you can push the limits of your instrument before the problem occurs, and then feel a little more comfortable playing it with the knowledge of having control over it. I like to think of it as getting to know your instrument, but I could be totally off the mark.